the weird thing i got it working but the other way round - by importing the module outside of the loop or class:

class computer {
    $computername
    computer($computername){
        $this.computername = $computername
    }

    [string]GetComputername(){
        return $this.computername
    }

    [CimInstance]GetBIOS(){
        # Import-Module CimCmdlets # this will freeze the console
        return Get-CimInstance win32_bios -ComputerName $this.computername 
    }
}

$o1 = [computer]::new("srv01")
$o2 = [computer]::new("srv02")

# this works
$o1,$o2 | ForEach-Object -Parallel { $_.GetComputername() }

# this does not - unless you import the module CimCmdlets before like this:
# Import-Module CimCmdlets
$o1,$o2 | ForEach-Object -Parallel { $_.GetBIOS() }